"Is" is for singular subject and for present tense, if the word "booked" is a past tense, why they use "is" instead of "was"? I agree that "leaving tonight" is a future but the action verb or word "booked" is a past tense.
This is present tense. There is an adjective "booked", and "Harold is booked". His name is on the list of passengers for that flight.
The adjective "booked" is based off of the past (or passive) participle, and the usage of the adjective is somewhat idiomatic. You would not normally say "Harold is a booked man" (though it might not be actually wrong to say that)
The verb "to book" describes an action, so you could say
Someone booked Harold on the flight (yesterday)
Someone has booked Harold on the flight.
Harold was booked on the flight (by someone)
Now this last expression is interestingly ambiguous. If read as passive it means "Someone booked Harold on the flight" (and unless we know otherwise I'd assume the booking is still valid) But if "booked" is understood as an adjective, then the past tense would mean that the booking is no longer valid. In other words, this sentence can have two very different meanings depending on how it is parsed. You would have to hope that some other context would make it clear.
Since "Someone books Harold on the flight" would not be a reasonable equivalent, this strongly suggests that "booked" in the original is an adjective and not the passive voice of the verb.
What part of speech is booked in the example? It is an adjective, not a verb. The word booked could be the simple past tense of to book, but here it is not.
Booked as a verb:
I booked a flight to Mexico City last night.
The chicken is cooked.
The flight is booked.
This seat is taken.